Brushless DC motors represent an attractive motor technology for many applications such as, for example, actuators. Advantageously, they have high operating efficiencies and high power densities. However, controlling these brushless DC motors is relatively complex, requiring dedicated controllers and multiple high-power semiconductor drivers.
One aspect of brushless DC motor control is regulating output torque. This is conventionally performed using motor current sensing. To summarize this technique, one or more current sensing resistors are placed in series with one or more of the motor windings. The voltage drop across the current sensing resistor(s) is measured, and indicates motor torque. Unfortunately, the use of current sensing resistors adds expense in terms of parts count, space required, cooling requirements, and energy efficiency.
A further complication of using brushless DC motors relates to starting and stopping them, particularly when under load. One previously used motor stopping technique involved maintaining high power on a particular winding combination to `lock` the motor in position. However, this consumes large amounts of power, generates excess heat, and is often ineffective if slippage occurs.
The above-described problems are multiplied when a brushless DC motor is included in another device such as, for example, an actuator. Static and dynamic loads resulting from the actuator application are imparted on the motor further complicating control thereof.
The present invention is directed toward solutions to the above-identified problems.